The present invention relates to a screw rotor for use in a screw pump that pumps fluid by rotation of a pair of screw rotors engaged with each other in the screw pump.
Japanese Unexamined Patent Application Publication No. 2008-38861 discloses a screw pump having a pair of single-thread screw rotors engaged with each other. The lead angle of the screw rotor on the suction side of the pump is larger than that on the discharge side of the pump, which is suitable for fluid transfer with increased amount of suction. However, the number of turns is less on the suction side of the screw rotor with large lead angle, which affects rotational balance of the screw rotor.
Japanese Examined Patent Application Publication No. 63-59031 discloses another screw pump having a pair of multiple-thread screw rotors engaged with each other. In such screw pump, the problem with rotational balance, as in the case of the single-thread screw rotor, does not occur.
However, when the number of turns on the discharge side of the screw rotor with small lead angle is increased, for example, in a double-thread screw rotor, its tooth thickness is approximately half of that of a single-thread screw rotor, under the condition of the same lead and the same fluid transfer volume. In such a case, sealing length between the inner surface of the rotor housing and the tooth is approximately half of that of a single-thread screw rotor, which leads to reduced sealing performance.
Japanese Unexamined Patent Application Publication No. 3-111690 discloses still another screw pump having a multiple-thread screw rotor on the suction side and a single-thread screw rotor on the discharge side. The multiple-thread screw rotor is located concentrically to the single-thread screw rotor. In such screw pump, the problem with rotational balance as in the case of the single-thread screw rotor does not occur, and the problem with sealing as in the case of the double-thread screw rotor does not occur, either.
However, in the screw pump disclosed in the publication No. 3-111690, since the multiple-thread screw rotor is axially spaced apart from the single-thread screw rotor, the volume of the fluid transfer space is increased at the part where the number of threads is changed. Such volume increase causes an expansion of the transferred fluid, which leads to inefficient pumping operation of the screw pump.
The present invention is directed to providing a screw rotor that prevents inefficient pumping operation of a screw pump while maintaining a good rotational balance of the screw rotor and ensuring sealing performance.